diff --git a/lib/std/cstr.zig b/lib/std/cstr.zig
index 4057d4b62b..9cb16b0ed3 100644
--- a/lib/std/cstr.zig
+++ b/lib/std/cstr.zig
@@ -28,7 +28,7 @@ test "cstr fns" {
 
 fn testCStrFnsImpl() void {
     testing.expect(cmp("aoeu", "aoez") == -1);
-    testing.expect(mem.len(u8, "123456789") == 9);
+    testing.expect(mem.len("123456789") == 9);
 }
 
 /// Returns a mutable, null-terminated slice with the same length as `slice`.
diff --git a/lib/std/fmt.zig b/lib/std/fmt.zig
index 97f2133d67..3729b7dc87 100644
--- a/lib/std/fmt.zig
+++ b/lib/std/fmt.zig
@@ -442,13 +442,11 @@ pub fn formatType(
             },
             .Many, .C => {
                 if (ptr_info.sentinel) |sentinel| {
-                    const slice = mem.pointerToSlice([:sentinel]const ptr_info.child, value);
-                    return formatType(slice, fmt, options, context, Errors, output, max_depth);
+                    return formatType(mem.span(value), fmt, options, context, Errors, output, max_depth);
                 }
                 if (ptr_info.child == u8) {
                     if (fmt.len > 0 and fmt[0] == 's') {
-                        const slice = mem.pointerToSlice([:0]const u8, @as([*:0]const u8, value));
-                        return formatText(slice, fmt, options, context, Errors, output);
+                        return formatText(mem.span(value), fmt, options, context, Errors, output);
                     }
                 }
                 return format(context, Errors, output, "{}@{x}", .{ @typeName(T.Child), @ptrToInt(value) });
diff --git a/lib/std/mem.zig b/lib/std/mem.zig
index 775e999e0b..a6c4d0057e 100644
--- a/lib/std/mem.zig
+++ b/lib/std/mem.zig
@@ -482,27 +482,21 @@ pub fn eql(comptime T: type, a: []const T, b: []const T) bool {
     return true;
 }
 
-/// Deprecated. Use `length` or `indexOfSentinel`.
-pub fn len(comptime T: type, ptr: [*:0]const T) usize {
-    var count: usize = 0;
-    while (ptr[count] != 0) : (count += 1) {}
-    return count;
-}
-
 /// Deprecated. Use `span`.
 pub fn toSliceConst(comptime T: type, ptr: [*:0]const T) [:0]const T {
-    return ptr[0..len(T, ptr) :0];
+    return ptr[0..len(ptr) :0];
 }
 
 /// Deprecated. Use `span`.
 pub fn toSlice(comptime T: type, ptr: [*:0]T) [:0]T {
-    return ptr[0..len(T, ptr) :0];
+    return ptr[0..len(ptr) :0];
 }
 
 /// Takes a pointer to an array, a sentinel-terminated pointer, or a slice, and
 /// returns a slice. If there is a sentinel on the input type, there will be a
 /// sentinel on the output type. The constness of the output type matches
-/// the constness of the input type. `[*c]` pointers are assumed to be 0-terminated.
+/// the constness of the input type. `[*c]` pointers are assumed to be 0-terminated,
+/// and assumed to not allow null.
 pub fn Span(comptime T: type) type {
     var ptr_info = @typeInfo(T).Pointer;
     switch (ptr_info.size) {
@@ -515,6 +509,7 @@ pub fn Span(comptime T: type) type {
         },
         .C => {
             ptr_info.sentinel = 0;
+            ptr_info.is_allowzero = false;
         },
         .Many, .Slice => {},
     }
@@ -541,7 +536,7 @@ test "Span" {
 /// the constness of the input type.
 pub fn span(ptr: var) Span(@TypeOf(ptr)) {
     const Result = Span(@TypeOf(ptr));
-    const l = length(ptr);
+    const l = len(ptr);
     if (@typeInfo(Result).Pointer.sentinel) |s| {
         return ptr[0..l :s];
     } else {
@@ -558,7 +553,7 @@ test "span" {
 
 /// Takes a pointer to an array, an array, a sentinel-terminated pointer,
 /// or a slice, and returns the length.
-pub fn length(ptr: var) usize {
+pub fn len(ptr: var) usize {
     return switch (@typeInfo(@TypeOf(ptr))) {
         .Array => |info| info.len,
         .Pointer => |info| switch (info.size) {
@@ -577,16 +572,16 @@ pub fn length(ptr: var) usize {
     };
 }
 
-test "length" {
-    testing.expect(length("aoeu") == 4);
+test "len" {
+    testing.expect(len("aoeu") == 4);
 
     {
         var array: [5]u16 = [_]u16{ 1, 2, 3, 4, 5 };
-        testing.expect(length(&array) == 5);
-        testing.expect(length(array[0..3]) == 3);
+        testing.expect(len(&array) == 5);
+        testing.expect(len(array[0..3]) == 3);
         array[2] = 0;
         const ptr = array[0..2 :0].ptr;
-        testing.expect(length(ptr) == 2);
+        testing.expect(len(ptr) == 2);
     }
 }
 
@@ -1867,8 +1862,13 @@ fn SubArrayPtrReturnType(comptime T: type, comptime length: usize) type {
     return *[length]meta.Child(meta.Child(T));
 }
 
-///Given a pointer to an array, returns a pointer to a portion of that array, preserving constness.
-pub fn subArrayPtr(ptr: var, comptime start: usize, comptime length: usize) SubArrayPtrReturnType(@TypeOf(ptr), length) {
+/// Given a pointer to an array, returns a pointer to a portion of that array, preserving constness.
+/// TODO this will be obsoleted by https://github.com/ziglang/zig/issues/863
+pub fn subArrayPtr(
+    ptr: var,
+    comptime start: usize,
+    comptime length: usize,
+) SubArrayPtrReturnType(@TypeOf(ptr), length) {
     assert(start + length <= ptr.*.len);
 
     const ReturnType = SubArrayPtrReturnType(@TypeOf(ptr), length);
diff --git a/lib/std/net.zig b/lib/std/net.zig
index b54803cd39..6d0daefdc0 100644
--- a/lib/std/net.zig
+++ b/lib/std/net.zig
@@ -352,7 +352,7 @@ pub const Address = extern union {
                     unreachable;
                 }
 
-                const path_len = std.mem.len(u8, @ptrCast([*:0]const u8, &self.un.path));
+                const path_len = std.mem.len(@ptrCast([*:0]const u8, &self.un.path));
                 return @intCast(os.socklen_t, @sizeOf(os.sockaddr_un) - self.un.path.len + path_len);
             },
             else => unreachable,
diff --git a/lib/std/os.zig b/lib/std/os.zig
index 24d78bec9a..127ada8fe5 100644
--- a/lib/std/os.zig
+++ b/lib/std/os.zig
@@ -1095,7 +1095,7 @@ pub fn createNullDelimitedEnvMap(allocator: *mem.Allocator, env_map: *const std.
 
 pub fn freeNullDelimitedEnvMap(allocator: *mem.Allocator, envp_buf: []?[*:0]u8) void {
     for (envp_buf) |env| {
-        const env_buf = if (env) |ptr| ptr[0 .. mem.len(u8, ptr) + 1] else break;
+        const env_buf = if (env) |ptr| ptr[0 .. mem.len(ptr) + 1] else break;
         allocator.free(env_buf);
     }
     allocator.free(envp_buf);
diff --git a/lib/std/special/c.zig b/lib/std/special/c.zig
index 1f11fabca0..e0c2636df1 100644
--- a/lib/std/special/c.zig
+++ b/lib/std/special/c.zig
@@ -47,7 +47,7 @@ fn strcmp(s1: [*:0]const u8, s2: [*:0]const u8) callconv(.C) c_int {
 }
 
 fn strlen(s: [*:0]const u8) callconv(.C) usize {
-    return std.mem.len(u8, s);
+    return std.mem.len(s);
 }
 
 fn strncmp(_l: [*:0]const u8, _r: [*:0]const u8, _n: usize) callconv(.C) c_int {
diff --git a/src-self-hosted/translate_c.zig b/src-self-hosted/translate_c.zig
index b33c758c38..8efc0e3746 100644
--- a/src-self-hosted/translate_c.zig
+++ b/src-self-hosted/translate_c.zig
@@ -4849,7 +4849,7 @@ fn transPreprocessorEntities(c: *Context, unit: *ZigClangASTUnit) Error!void {
                 }
 
                 const begin_c = ZigClangSourceManager_getCharacterData(c.source_manager, begin_loc);
-                const slice = begin_c[0..mem.len(u8, begin_c)];
+                const slice = begin_c[0..mem.len(begin_c)];
 
                 tok_list.shrink(0);
                 var tokenizer = std.c.Tokenizer{
diff --git a/test/stage1/behavior/misc.zig b/test/stage1/behavior/misc.zig
index 681f5be500..44a8334b5d 100644
--- a/test/stage1/behavior/misc.zig
+++ b/test/stage1/behavior/misc.zig
@@ -335,7 +335,7 @@ test "string concatenation" {
     comptime expect(@TypeOf(a) == *const [12:0]u8);
     comptime expect(@TypeOf(b) == *const [12:0]u8);
 
-    const len = mem.len(u8, b);
+    const len = mem.len(b);
     const len_with_null = len + 1;
     {
         var i: u32 = 0;